Safe and effective antiviral drugs are not currently available for the prevention and treatment of Polyomavirus-associated nephropathy (PVAN). PVAN results from reactivation of the polyomavirus BK (BKV) in renal allograft recipients. Seven percent of transplant patients are affected within five years of transplantation. If not properly treated, graft function is lost in over 50 percent of PVAN cases. Current therapies to limit virus replication rely on immunosuppression reduction, which risks acute rejection of the graft. A BKV antiviral would be of great value in replacing or complementing existing therapies, particularly in cases where BKV is refractory to reduced immunosuppression or when PVAN occurs in combination with acute graft rejection. The primary goal of this application is to identify therapeutic compounds that limit replication of BKV by inhibiting specific host cell pathways. Recent studies have shown that members of the polyomavirus family induce a DNA damage response in the host cell and require the damage response for optimal virus replication. This response is similar to the DNA damage response induced by gamma and ultraviolet radiation, which arrests the cell cycle and allows time for DNA repair. Preliminary results are presented demonstrating that BKV- infection of cultured human renal proximal tubule epithelial cells activates at least two protein kinases in DNA damage response pathways. The relevance of these in vitro findings to clinical PVAN is demonstrated by immunohistochemical evidence of a DNA damage response in PVAN biopsies. In addition, proof of principle is provided by an inhibitor of ATM, a major protein kinase in DNA damage response pathways, that exhibits a concentration-dependent inhibition of BKV protein expression and DNA replication in infected cells. It is hypothesized that BKV-infection of human renal proximal tubule epithelial cells induces cellular DNA damage response pathways that are required for optimal virus replication and that these required pathways provide potential targets for antiviral therapy. Cultured human renal proximal tubule epithelial cells will be used to determine if small molecule inhibitors of th DNA damage response that are in preclinical development or clinical trials are also effective in inhibiting BKV replication. The role of specific DNA damage response proteins in BKV replication will also be defined and their clinical relevance confirmed by immunohistochemical staining of PVAN biopsies. If DNA damage response inhibitors that are currently in clinical trials are also found to be effective in limiting replication of BKV, it may be possible to reposition the for the prevention and treatment of PVAN.